Defining Abrupt Climate Change

Weather or Climate?

What is weather? Weather is the state of the atmosphere at a particular time, as defined by variables such as temperature, precipitation or winds.

What is climate? Climate is the "average weather" over a period ranging from months to thousands or millions of years. Climate Change is a departure from the expected average weather patterns, which are also known as the "climate normals".

One way to distinguish between weather and climate is that the climate of your hometown will determine how many sweaters you have in your closet. The weather will determine whether you should be wearing a sweater right now.

Definitions of abrupt climate change are still evolving in this new area of scientific research. The National Research Council proposes that "an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause" (NRC, 2002). The U.S. Climate Change Science Program defines it as: "a change in the climate (for example, in temperature or precipitation) that takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems." Abrupt climate changes may occur over a region, a hemisphere, or the entire globe.

For a visual analogy of an abrupt climate change, imagine a landscape with two valleys and a ball sitting in one of these valleys (Figure 1). A gradual push is given to the ball and it begins to roll up the hill. If the push is not strong enough, the ball stops midway up the hill and rolls backward to its original position. With a stronger push, the ball rolls up the hill and, suddenly, the ball tops the hill and rolls down the other side into the second valley. An abrupt change to a new stable state has occurred.
The ball, which represents some aspect of the climate system, moved rapidly from one state to another: the first valley to the second valley. This happened due to a forcing, the push, being applied gradually to the ball. Abrupt changes can also occur spontaneously or due to chaotic (random) behavior in the climate system. This is illustrated in the animation as the ball continues to roll around the higher valley and eventually its momentum causes it to spontaneously top the hill and return to the first stable state.

Figure 1. Animation depicting a ball moving from one stable state to another due to a forcing. In this example, gravity is the primary physical force maintaining the stable state. In climate systems, the physics involved can be far more complex.

Using paleoclimate records and computer models, scientists have identified many "valleys", or stable states, in which the climate system can reside, and "pushes" that cause it to move from one state to another. Our knowledge of the mechanisms of abrupt climate change is far from complete and is constantly being improved. One set of mechanisms crucial to the existence of abrupt climate change are positive feedbacks. These feedbacks are discussed in the next chapter.